The present invention relates to a bioelectrical impedance measuring system that measures and indicates the impedance of biological subjects, such as human extremities. More particularly the invention relates to a bioelectrical impedance measuring system that indicates a basal impedance value of the measured subject and also indicates variations in the basal value.
Bioelectrical impedance measurements are well known in the prior art. More particularly, such measurements are conducted for well established clinical purposes such as the measurement of fluid volume changes, estimation of cardiac output, and detection of deep venous thrombosis and peripheral vascular disease. The bioelectrical impedance of a segment of tissue or an extremity of the human body is a measure of the opposition to the flow of an electrical current. When blood pulses into a segment of the body, the electrical impedance of that body segment is changed by the varying amount of blood present. The presence of a slightly increased amount of blood will reduce the impedance of the tissue segment; and, conversely, when blood flows out of the tissure segment, the impedance increases. The bioelectrical impedance may be measured in a variety of ways, such as by applying a minute radio-frequency current to the tissue being monitored and by sensing the alterations that occur in the signal detected from the tissue through use of an instrumentation system. Typical systems are described in U.S. Pat. Nos. 2,111,135; 2,184,511 and 3,149,677 to Bagno, and 3,340,867 to Kubicek et al.
A report titled "Bioelectrical Impedance Measurements as a Method of Screening for Peripheral Vascular Disease and Deep Venous Thrombosis" by Allan F. Pacela dated May 25, 1971, includes a comprehensive discussion of the background and utilization of bioelectrical impedance measurements. In addition the report discloses a number of instrumentation systems utilized in the prior art for bioelectrical impedance measuring.
One such instrumentation system is discussed in Section 5.5 of the Pacela report under the subheading of "Impedance Ratio Systems". FIG. 10 of the Pacela report and the above-identified section generally disclose an impedance ratio system wherein an excitation signal is coupled to the subject to be measured by excitation electrodes, and wherein a resulting subject output signal is sensed by receiver electrodes. The subject output signal is electronically subtracted from a reference level derived from the excitation source. In such a system, it is envisioned that the subject excitation signal be derived from a constant current source, and that the level of the constant current be adjusted so that the voltage across the sensing electrodes is always the same. This is accomplished by a balance control which is adjusted so that the d.c. level of the system output is at a known level, preferably a null condition. In that event the voltage drop across the receiver electrodes is at a known level and the system output represents a ratio of impedances (.DELTA.Z.sub.0 /Z.sub.0) which is the desired function. In that event Z.sub.0 is the steady "basal" value of impedance of the body segment, and .DELTA.Z.sub.0 represents any variation in the basal value.
The impedance ratio system described hereinbefore has certain advantages in bioelectrical impedance measuring applications. However, a major disadvantage in the system is the difficulty of adjusting the excitation function to achieve a null condition throughout the normal range of subject impedance values. For example, the value of Z.sub.0 can span a range of more than 10 to 1. Consequently, when maintaining currently accepted values of allowable subject excitation under present safety requirements, the excitation current in the impedance ratio system must be adjusted to a point where the excitation elements and controls of the system contribute appreciably to the noise level in the system. In addition, as is mentioned in the report, the system must be frequently rebalanced and thus is less useful in clinical applications. Finally, if a bilateral measurement is to be made, i.e. where two extremities of a subject are excited in series by the same excitation function, the electrode technique to achieve identical scale factors for each of the two receiver channels becomes hypercritical. Consequently the effective use of the system in such applications is nearly impossible.